Dear experts,
I wanted to signal a peculiar, unexpected behaviour of 'apply'. It is not a
bug, it is per spec, but it is so counterintuitive that I thought it could
be interesting.
I have an array, let's say "test", dim=c(7,5).
> test <- array(1:35, dim=c(7, 5))
> test
[,1] [,2] [,3] [,4] [,5]
[1,] 1 8 15 22 29
[2,] 2 9 16 23 30
[3,] 3 10 17 24 31
[4,] 4 11 18 25 32
[5,] 5 12 19 26 33
[6,] 6 13 20 27 34
[7,] 7 14 21 28 35
I want a new array where the content of the rows (columns) are permuted,
differently per row (per column)
Let's start with the columns, i.e. the second MARGIN of the array:
> test.m2 <- apply(test, 2, sample)
> test.m2
[,1] [,2] [,3] [,4] [,5]
[1,] 1 10 18 23 32
[2,] 7 9 16 25 30
[3,] 6 14 17 22 33
[4,] 4 11 15 24 34
[5,] 2 12 21 28 31
[6,] 5 8 20 26 29
[7,] 3 13 19 27 35
perfect. That was exactly what I wanted: the content of each column is
shuffled, and differently for each column.
However, if I use the same with the rows (MARGIIN = 1), the output is
transposed!
> test.m1 <- apply(test, 1, sample)
> test.m1
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,] 1 2 3 4 5 13 21
[2,] 22 30 17 18 19 20 35
[3,] 15 23 24 32 26 27 14
[4,] 29 16 31 25 33 34 28
[5,] 8 9 10 11 12 6 7
In other words, I wanted to permute the content of the rows of "test", and
I expected to see in the output, well, the shuffled rows as rows, not as
column!
I would respectfully suggest to make this behavior more explicit in the
documentation.
Kind regards,
Luca Nanetti
--
______________
Luca Nanetti, MSc, MRI
University Medical Center Groningen
Neuroimaging Center Groningen
Groningen, The Netherlands
Tel: +31 50 363 4733
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